1. Field of the Invention
The present invention relates to an X-ray analyzing apparatus for irradiating exciting X-rays to a product to be tested such as a semiconductor wafer to analyze fluorescent X-rays generated from the product by EDX (Energy Dispersive X-ray Spectrometry) and to measure surface roughness, thickness of film on the surface, density of the surface and other characteristics of the product by analyzing reflectivity of the X-rays. The present invention also relates to an X-ray irradiation angle setting method for setting an irradiation angle of X-rays to the product.
2. Description of Related Art
Hitherto, there has been known a total reflection X-ray fluorescence analyzing apparatus for irradiating X-rays to a product to be tested such as a semiconductor wafer having an optically flat plane at a low incident angle to detect fluorescent X-rays from a sample adhering on a surface of the product. It allows information only in the vicinity of the surface of the product to be obtained with a high S/N (signal to noise) ratio by totally reflecting the exciting X-rays on the surface of the product.
Further, there has been proposed a monochromic total reflection X-ray fluorescence analyzing apparatus (Japanese Patent Application 1-272124(1989)) for irradiating a characteristic X-ray which is generated from an anode of an X-ray source to a product to be tested after separating X-rays of a single characteristic by spectroscopic apparatuses, e.g., a spectroscopic crystal, a slit and others. Because it allows background noise to be reduced by separating as the monochromic exciting X-rays and the limit for detecting the trace element to be improved, it is rapidly spreading, in particular, in the field for detecting contaminants caused by a trace metal on a semiconductor wafer for LSIs.
FIG. 9 is a structural diagram showing an example of the conventional X-ray fluorescence analyzing apparatus. This X-ray fluorescence analyzing apparatus comprises an X-ray tube 71 for generating an X-ray beam B1, a monochromator crystal 72 for separating an X-ray beam B2 composed of a single characteristic X-ray from the X-ray beam B1, a collimator 73a for blocking other characteristic X-rays, a moving table 74 for supporting a product to be tested 70 (hereinafter referred to as "test product") such as a semiconductor wafer, a collimator 73b for blocking scattered X-rays other than the X-ray beam B2, an X-ray counter 81 for measuring the intensity of the X-ray beam B2, a table controller 75 for setting three-dimensional position of the moving table 74 and an angle thereof to the X-ray beam B2, a detector 76 for detecting fluorescent X-rays B3 generated from the test product 70, a pre-amplifier 77 for converting into stepped voltage pulses having a pulse height of the time integral value of the charge pulse outputted from the detector 76, a proportional amplifier 78 for shaping the waveform as a pulse having a pulse height proportional to a width of a leading edge of a voltage pulse outputted from the pre-amplifier 77, a pulse-height analyzer 79 for measuring a counting rate of each peak value outputted from the proportional amplifier 78, a data processor 80 for processing data measured by the wave-height analyzer 79 and the X-ray counter 81 and for issuing commands to the table controller 75, and others. Information about the vicinity of a surface of the test product 70, e.g. information on concentration of trace contaminants, can be obtained by irradiating the X-ray beam B2 to enter the surface of the test product 70 at an angle to the surface where the beam totally reflects, e.g. an angle of about 0.06 degrees.
A method for setting an angle for irradiating the energy beam to the test product 70 in the X-ray fluorescence analyzing apparatus shown in FIG. 9 has been proposed in Japanese Patent Application No. 2-400231 and others.
FIGS. 10A through 10E are flow diagrams showing a method for setting an irradiation angle in the apparatus of FIG. 9. At first, as shown in FIG. 10A, the X-ray beam B2 outputted from an X-ray source 82 composed of either the X-ray tube 71, or the X-ray tube 71 and the monochromator crystal 72, is caused to directly enter the X-ray counter 81 to measure the intensity of the X-ray beam B2 to store the initial value of intensity in a memory or the like within the data processor 80. At this time, the test product 70 is set at a position where the test product will not block the X-ray beam B2, to be inclined slightly with respect to the X-ray source 82.
Next, as shown in FIG. 10B, the table controller 75 drives the moving table 74 to raise the test product 70 gradually, while the intensity of X-ray is measured by the X-ray counter 81. When the test product 70 gradually blocks the X-ray beam B2 and the intensity of the X-rays outputted by the X-ray counter 81 reaches a reference value, e.g. a half of the initial value of intensity, the test product 70 is stopped from rising.
Then, as shown in FIG. 10C, while on the basis of a command from the data processor 80, the inclination angle of the test product 70 is changed so that the inclination approaches the horizontal direction, the intensity of the X-ray beam is measured by the X-ray counter 81, and when the intensity outputted by the X-ray counter 81 reaches a relative maximum, the angular displacement of the test product 70 is stopped. When the relative maximum value of the intensity of the X-ray beam at this time is greater than the reference value, it is determined that the test product 70 is not parallel with the X-ray beam B2 and the test product 70 is gradually raised in FIG. 10D similarly to FIG. 10B to control the test product 70 in vertical position so that the intensity of the X-ray beam coincides with the reference value.
Next, as shown in FIG. 10E, the same as in FIG. 10C, the test product 70 is controlled to be gradually angularly displaced so that the intensity of the X-ray beam reaches the relative maximum value.
Raising and angularly displacing the test product 70 is thus repeated, and when the reference value brought about by the raising operation becomes equal to the relative maximum value brought about by the angular displacement, the traveling direction of the X-ray beam B2 is set parallel with the surface of the test product 70. After that, the table controller 75 drives the moving table 74 so that the X-ray beam B2 is irradiated at an incident angle of a predetermined total reflection angle. The setting of the irradiation angle of the X-ray beam B2 to the test product 70 is thus completed.
Because the exciting X-ray beam itself is thus used for the adjustment and the position and the angle can be adjusted accurately, the elapsed fluctuation error of the X-ray beam axis can be absorbed by the adjustment even if it occurs. Further, even if the peripheral edge of the test product 70 is curved and drops by its own weight, the part around the center from which the fluorescent X-rays are generated is correctly adjusted. Further, it allows a desirable incident angle to be set to the upper most face of a semiconductor wafer when a semiconductor wafer on which patterns have been formed is measured as a test product.
Meanwhile, beside the measurement using such fluorescent X-rays, there has been developed a method for measuring surface information, e.g. surface roughness, thickness of thin film, density and others, of a test product by irradiating X-rays at a very low angle and by directly measuring the intensity of X-rays reflected by the test face.
The measurement of surface roughness utilizes the nature that the rougher the surface of the test product, the lower the X-ray reflectivity is, and the incident angle of X-ray must be accurately set.
The measurement of thickness of thin film utilizes an interference caused by a difference of optical paths of X-rays reflected by the surface of the film and X-rays which pass through the film and are reflected by the surface of a test product, to measure a period of reflectivity of X-rays which changes according to the incident angle of the X-rays. The incident angle of the X-rays must be set at high precision also in this measurement.
The measurement of density specifies the density of the surface of a test product by measuring a critical incident angle on the surface of the test product and the X-ray incident angle must be set at high precision also in this measurement.
The apparatus is arranged such that an X-ray detector is disposed in the direction in which the X-rays are reflected in such measurement of reflectivity of X-rays, and the apparatus shown in FIG. 9 may be also used.
However, the apparatus shown in FIG. 9 has a problem that because the intensity of X-rays is measured by using the X-ray counter 81 while alternately controlling the height and angle of the moving table 74, it takes time for the measurement.
Further, when the face of the test product 70 is not an ideal plane and is deformed by warp or distortion, the X-ray irradiation angle at the actual irradiation point deviates from the setting angle of the moving table 74, containing a deformation error. The dependency on angle of the X-ray reflectivity is an important measurement item in the measurement of surface roughness, thickness, density and others of the test product 70, and the measured result is largely swayed if there is even a slight error in the X-ray irradiation angle.